1. Field of the Invention
The invention relates to a multilayer circuit board used for an electrical circuit board or the like, and more particularly to a structure of a conductive layer for electric wiring in the multilayer circuit board.
2. Background of the Invention
In a conventional multilayer circuit board used for electric circuit boards or the like on which a semiconductor device such as a semiconductor integrated circuit device is mounted, an insulating layer composed of ceramics, e.g. alumina and a conductive layer for electric wiring composed of a metal having a high melting point, e.g. tungsten are overlaid alternatively to produce a multilayer circuit board.
In the conventional multilayer circuit board, a signal wiring conductor in the conductive layers for electric wiring is formed to have a strip line structure. In other words, wholly patterned ground layers are formed on and underneath the conductive layer formed as the signal wiring conductor, via insulating layers.
For the purpose of stabilizing the power supply to the semiconductor device mounted on the multilayer circuit board, a capacitance has been generated between the ground layer and the conductive layer for power supply by overlaying the wholly patterned ground layer formed in the multilayer circuit board and the conductive layer functioning as a power wiring conductor alternatively.
Furthermore, for adapting to electrical signals to be processed by the multilayer circuit board becoming increasingly faster, the multilayer circuit board has been produced in the following manner: first forming an insulating layer of a polyimide resin or epoxy resin having a comparably small relative dielectric constant of 3.5 to 5.0 in place of the alumina ceramics with a relative dielectric constant of about 10; forming a conductive layer for electrical wiring formed of a copper (Cu) on the insulating layer by the use of a thin-film formation technique employing the vapor phase deposition methods such as vacuum evaporation or sputtering; and forming a fins wiring pattern by the photolithography to thereby overlay the insulating layer and the conductive layer. In this way, a multilayer circuit board with high density and excellent function, as well as capable of operating at high speed has been produced.
The above-mentioned multilayer circuit board in which the insulating layer is formed of a polyimide resin or epoxy resin, however, has a problem that even when the not-yet-reacted component in the resin and the water absorbed in the resin evaporate at the curing step in the overlaying process, a path for dissipating the not-yet-reacted component and the water to the outside air is cut out because of the solid plane formed on the resin layer, causing the solid plane to expand. In view of this problem, the ground layer has been formed to be mesh-like layer, so that it becomes difficult to make a capacitance which used to be generated in the conventional multilayer circuit board using the ceramics insulating layer, by overlaying the wholly patterned ground conductive layer and the conductive layer for power wiring. In this type of multilayer circuit board, therefore, a measure of mounting a chip capacitor on the multilayer circuit board has been taken.
In the case of forming the circuit board of multilayer structure using the conventional insulating film of polyimide or epoxy resin, however, the ground layer has been formed into a mesh-like layer. The conductive layer for signal wiring interposed between the mesh-like ground layers via the insulating layers from top and bottom thereof is arranged so as to oppose to both a portion formed with a metal layer forming the mesh-like ground layer and a portion clear of the metal layer. As a result, the characteristic impedance of the signal wiring conductor varies depending on the positional relationship between the signal wiring conductor and the ground layer
in this type of conventional multilayer circuit board, since the high-speed electrical signal, which has increasingly become faster according to the speed up of the semiconductor device in recent years, passing through the signal wiring conductor propagates through the signal wiring conductor having variable characteristic impedance, a problem arises that the signal is partly reflected and the input electrical signal is not correctly transmitted to an output side, causing erroneous operations of the electrical circuit and the semiconductor device.
Moreover, another problem relating to the power supply to the semiconductor device mounted on the multi layer circuit board has been recognized that the power required for power supply to the semiconductor device cannot be transmitted due to the inductance in the wiring from the chip capacitor for stabilizing the power supply to the semiconductor device, and the path length for power supply.
For solving the disadvantages in the conventional multilayer circuit board, in Japanese Unexamined Patent Publication JP-A 9-18156(1997) and the research report by University of Arkansas in U.S. *Modeling and Experimental Verification of the Interconnected Mesh Power System (IMPS) MCM Topologyy IEEE TRANSACTION ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY--PART B. VOL. 20, NO. 1, FEBRUARY 1997, p 42-49, for example, solutions for eliminating the nonuniformity in the characteristic impedance of the signal wiring portion which is the problem in the mesh-like ground structure are disclosed. In these publications, the multilayer circuit board is formed in such a manner that: the power wiring conductor, ground wiring conductor and signal wiring conductor are formed in the same layer; the signal wiring conductor is arranged between the power wiring conductor and the ground wiring conductor to form a coplanar line structure; and then the coplaners are multilayered at the position of torsion.
In the multilayer circuit board employing the above-mentioned coplaner wiring structure, however, power is supplied from the chip capacitor to the semiconductor device via the power wiring conductor and the ground wiring conductor because there is not any capacitance made between the power wiring conductor and the ground wiring conductor. Accordingly, there arose a problem that the inductance components of the power wiring conductor and the ground wiring conductor, and the path for power supply increase, so that stable power supply becomes impossible to cause the erroneous operation of the semiconductor device.